Matthew says each time period has fourteen generations. But the first and third periods list only thirteen. A traditional Jewish and Old Testament method would count King David in both the first and second groups. It would include Jesus in the third group.
This suggests Matthew is probably emphasizing the gematria (a system where letters represent numbers). The letters in the Hebrew word dawid (meaning "David") add up to fourteen. Matthew is emphasizing Jesus’ credentials as the Messiah (1:1).